Copper (Cu) is one of the promising conductors for next generation ultra-large scale integration (ULSI) metallization due to Cu's low resistivity and high electomigration resistance. Successful implementation of copper metallization requires a suitable means of filling trenches and vias with Cu. Fast depositing and good gap filling have been achieved using electrochemical deposition (ECD) techniques followed by chemical mechanical polishing (CMP) for planarization. However the major drawback of these ECD techniques is limited by additives (including polyethylene glycol (PEG), polyethylene imine (PEI), or polyvinylalcohol) mass transfer, which results in poor gap filling in high aspect ratio, i.e. relatively deep, narrow width trenches damascene structures. For such high aspect ratio trenches, Cu deposition closes off the upper portion, or throat, of the trench, or via, before the trench is completely filled resulting in voids or seams in the Cu filled trench which adversely affects performance especially in multiple layers of stacked such Cu filled trenches.
"Suppressing" and "brightening" agents are two types of organic additives commonly used in electroplating along with chloride. Suppressors, also known as "carriers" or "levelers" depending on specific functionality, are macromolecule deposition inhibitors that tend to adsorb over the wafer or semiconductor substrate and reduce local deposition rates, while brighteners are organic molecules that tend to improve the specularity (or reflectivity--i.e. smooth films appear bright while rough films appear dark or hazy) of the deposit by reducing both surface roughness and grain-size variation. Brighteners interact with suppressors and compete for surface adsorption sites and locally accelerating deposition rates. For acid copper electroplating, most commercial electroplating mixtures use three organic components, i.e.: brighteners; suppressors which comprise levelers and carriers; and chloride ions which adsorb at the cathode during plating.
The general chemical characteristics of brighteners are water soluble salts of organic acids containing a mercapto or thiol functional group. A typical example would be: EQU H--S--C--C--C--SO.sub.3
present at 1 to 10 ppm concentration. Brighteners adsorbs strongly on Cu metal during plating and participates in the charge transfer reaction. They determine Cu growth characteristics with major impact on ductility, smoothness, and hardness. Brighteners are the least stable of all additive components and are subject to oxidation by air (oxygen), electrochemical oxidation at the anode, and catalytic decompositon at the Cu surface. By-products of brighteners are often detrimental to deposit properties and control of brighteners during electroplating is relatively important and is the subject of most acid-Cu additive control.
The general chemical characteristics of levelers are a high molecular weight monomer or polymer with both sulfonic acid and nitrogen containing functional groups and are usually present at 10 to 100 ppm concentrations. The nitrogen containing groups are protonated in acid solution and adsorb strongly on Cu to inhibit plating. The exact chemical composition varies by supplier and application requirements. Levelers reduce Cu growth at edges and protrusions to yield a smoother final surface. They increase polarization resistance at high growth areas by either: inhibiting growth proportional to mass transfer at localized sites; or adsorbing more selectively at more cathodic (fast growing) sites. Levelers are relatively stable compared to brighteners.
The general chemical characteristics of carriers are oxygen containing polymers containing no sulfur or nitrogen groups. Typical examples are polyethylene glycol or polyoxyethylene glycol of from 3000 to 8000 molecular weight. Carriers adsorb during Cu plating to form a relatively thick monolayer film at the cathode and moderately polarize Cu deposition by preventing diffusion of Cu.sup.2+ ions to the surface. A carrier alone, or with Cl.sup.-, yields good thickness distributions but poor fill and deposit properties. Carriers effect on plating is stable over a wide range of concentrations, from 10 to 1,000 ppm, and molecular weights from 800-1,000. Carriers are gradually broken into lower molecular weight fragments at both the anode and the cathode and tend to lose polarization effectiveness below a molecular weight of 750.
Chloride, as HCl, is an additive for nearly all commercial additive electroplating systems and is present at bath concentrations of 30 to 100 ppm. It adsorbs at both the cathode and anode and modifies adsorption properties of the carrier to influence thickness distribution. It accumulates in anode film and increases anode dissolution kinetics. Chloride does not decompose or complex irreversibly with other bath components and the bath chloride concentrations may shift inversely with the anode film chloride level.
For ultralarge-scale integration (ULSI) interconnect applications, the composition and concentrations of brighteners and suppressors are selected such that brightener surface concentration dominates on the interior surfaces of trenches and vias. Local deposition rates are thus supressed at the top of topographical features relative to the insides, leading to the desired "bottom-up" deposition and void-free metal filing of the trenches and vias. In order to have sufficient Cu to cover the step height and to improve planarity before CMP, the thickness of the Cu layer has to be increased. However, increased thickness of the Cu layer leads to poor uniformity and lower CMP throughput.
U.S. Pat. No. 5,662,788 to Sandhu et al. describes a method for forming a metallization layer which uses a single electrodeposition step to form both the metallization layer and fill the contact vias. Metal ions are electrodeposited by applying a bi-polar modulated voltage having a positive duty cycle and a negative duty cycle to first and second layers of material and the solution. The voltage and surface potentials are selected such that the metal ions are deposited on the portions of the second layer that define the layout of the metallization layer and the contact vias.
U.S. Pat. No. 5,723,387 to Chen describes a self contained unit for forming copper interconnection structures on semiconductor (SC) substrates. The unit has an enclosed chamber with a plurality of apparatus for performing wet processes and provides a way of reducing the number of times the wafer is transferred between the wet process steps that require less environmental cleanliness and dry, very clean process steps.
U.S. Pat. No. 5,821,168 to Jain describes a process for forming a semiconductor device in which an insulating layer is nitrided and then covered by a thin adhesion layer before depositing a composite copper layer. This process eliminates the need for a separate diffusion barrier since a portion of the insulating layer is converted to form a diffusion barrier film and the adhesion layer reacts with the interconnect material resulting in strong adhesion between the composite copper layer and the diffusion barrier film formed on the insulating layer. After a copper seed layer is deposited by physical vapor deposition over the adhesion layer using a collimated sputtering chamber, the substrate is taken to an electroplating system where 6,000-15,000 .ANG. of copper is plated over the copper seed layer forming a composite copper layer with the copper seed layer indistinguishable from the plated copper layer. CMP then removes the composite copper layer overlying the uppermost surface of the insulating layer.
U.S. Pat. No. 5,472,592 to Lowery describes an apparatus for electrolytic plating of a substrate within an electrolytic bath. The apparatus includes a tank structure for containing an electrolyte and an anode. A shaft is rotatably mounted within the tank to rotate about a first axis. An arm is mounted on the shaft, and a fixture for receiving the substrate is rotatably mounted on the arm to rotate about a second axis, so that the substrate is both revolved about the first axis and rotated about the second axis. Electrical contact is maintained between the rotating substrate and a stationary power supply for plating.
U.S. Pat. No. 4,169,018 to Berdan et al. describes a copper electroplating method typically used on a carrier such as aluminum. The carrier surface is pretreated and copper foil is electroplated unto the treated carrier surface in a single-step process comprising an acidic plating bath containing copper, nitrate and fluoride ions which can be operated at a single-current density.